A. Field of the Invention
The invention relates to a semiconductor processing equipment, and especially to insulating sleeves wrapping around inlet pipes of a horizontal driven field oxidation tube to prevent water vapor from accumulating in the inlet pipes and causing unstable growth rate of oxide in the process of dry oxidation.
B. Description of the Prior Art
A conventional semiconductor processing equipment includes four parts: source cabinet 11, furnace module 12, load station 13, and controller 14, as shown in FIG. 1. Furnace module 12 contains horizontal driven field oxidation tube 101 for processing wafers. The horizontal driven field oxidation tube 101 is surrounded with heating element 102 for heating wafers inside. At the closed end of the horizontal driven field oxidation tube 101, there are several inlet pipes for gas injection and thermocouple insertion. Load station 13 is a wafer transfer system which can load wafers 103 to horizontal driven field oxidation tube 101. Wafers 103 are placed on top of boat 104 to be delivered by paddle 105 into horizontal driven field oxidation tube 101. Source cabinet 11 contains gas piping (not shown in FIG. 1) and a mass flow controller (not shown in FIG. 1). At the closed end of horizontal driven field oxidation tube 101, there is a gas injector 106 which injects high purity gases into quartz tube 101 such as, O.sub.2, H.sub.2 O, HCL, TCa, TDCE, and so on. Controller 14 controls the factors of processing wafers such like, temperature, timing, air, and recipe.
In a horizontal driven field oxidation tube, normally the procedures of dry oxidation and wet oxidation are processed interchangeably. The procedure of dry oxidation injects oxygen to the tube while the procedure of wet oxidation injects oxygen and hydrogen to the tube. The procedure of wet oxidation is applied when thin layers of oxide, such as 700 Angstroms, is desired. In contrast, the procedure of dry oxidation is applied when a thicker layer of oxide is required, such as 5000 Angstroms. For the same gas sources, the former procedure takes about 7 minutes while the latter procedure 2 hours.
As observed, if the two procedures are processed interchangeably, the growth rate of oxide becomes very unstable. For instance, if the procedure of wet oxidation is processed first and immediately followed by another wet oxidation, then the problem is not obvious. The reason is because the error is relatively small comparing to 5000 Angstroms. Nevertheless, if the procedure of wet oxidation is processed first and immediately followed by the procedure of dry oxidation, then the growth rate of oxide becomes very unstable. Suppose the procedure of dry oxidation is expected to form 700 Angstroms oxide in 7 minutes. Now it forms 800 Angstroms oxide.
The phenomenon of the unstable growth rate of oxide has been noticed for a long time. However, there is no solution to this problem because the cause has not been found yet. As the problem occurs, the only solution is trying to produce the desired thickness of oxide by adjusting the duration of gas injection. For instance, if the procedure of dry oxidation is supposed to form 700 Angstroms oxide in 7 minutes but now forms 800 Angstroms oxide instead. To form 700 Angstroms oxide, the duration of gas injection must be adjusted to 6 minutes. However, in next run, 6 minutes of gas injection may not form 700 Angstroms oxide but only 600 Angstrom oxide. Consequently, the duration of gas injection must be adjusted each time using trial and error.
What is puzzling is the cause for the unstable growth rate of oxide. There are several reasons to consider. Is it due to the steam remains in the tube when cleaning or the temperature not optimized yet? The trial and error method as mentioned above is very tedious. The unstable growth rate of oxide causes a great trouble for semiconductor fabrication that demands exact value of thickness of oxide.